The present invention relates generally to space division switching networks, and more particularly to a method for installing expansion switching stages to an existing switching network to accommodate an increasing volume of traffic.
As is well known in the telephone switching art, a strictly nonblocking N by N three-stage switching network can be realized when the number of second-, or center-stage switches is equal to 2n-1, (where n is the number of inlets to each of the individual switches of the first and third stages) provided that each individual switch is nonblocking. Therefore, the number of center-stage switches for a strictly nonblocking 2N by 2N three stage network is equal to 4n-1.
FIG. 1 illustrates a conventional, strictly nonblocking N by N three stage switching network in which the inlets and outlets are partitioned into N/n groups of n inlets and n outlets each. The first switching stage 1 is N/n units of n.times.(2n-1) of matrix switches 1.sub.1 through 1.sub.N/n where each one of the (2n-1) outputs is connected to one of the (2n-1) units of matrix switches 2.sub.1 through 2.sub.2N-1 of the second, center stage 2. The third stage 3 consists of N/n units of (2n-1).times.n matrix switches 3.sub.1 through 3.sub.N/n. All center stage switches are of N/n by N/n configuration that provide connections from any first stage switch 1 to any third stage switch 3. If it is desired to double the capacity of the N by N network of FIG. 1, the resultant, strictly nonblocking 2N by 2N switching network would be of a configuration as illustrated in FIG. 2 in which the inlets and outlets are partitioned into N/n groups of 2n inlets and 2n outlets each. The first stage of the 2N by 2N three stage switching network is made up of N/n units of 2n.times.(4n-1) matrix switches 4.sub.1 through 4.sub.N/n where each one of the (4n-1) outputs leads to one of the (4n-1) units of center stage switches 5.sub.1 through 5.sub.4N-1. The third stage 6 comprises N/n units of (4n-1).times.2n switches 6.sub.1 through 6.sub.N/n. As in FIG. 1, all center-stage switches of the FIG. 2 arrangement are of N/n by N/n configuration to provide connections from any first stage switch 4 to any third-stage switch 6. It is seen that center-stage switches 5.sub.1 through 5.sub.2n-1 correspond to center-stage switches 2.sub.1 through 2.sub.2n-1 of FIG. 1 and therefore, it is not necessary to replace them with new switches. Because of the substantially different structures and wiring configurations, all the first and third stage switches of the strictly nonblocking N.times.N network must be replaced with the first and third stage switches of a strictly nonblocking 2N.times.2N network. However, the replacement would cause interruptions of service.